The present invention relates to a single-vision spectacle lens to correct eyesight and particularly, to an aspherical lens having a prismatic power to correct heteophoria (more conventionally known as heterophoria) of an eye.
A spectacle lens for correcting heteophoria (visual axes are deviated during a resting period) has a prismatic power. A conventional aspherical lens produces the prismatic power by tilting a back surface (an eye side) with respect to a front surface (an object side).
FIGS. 113 and 114 show an example of a conventional spectacle lens having a prismatic power; FIG. 113 is a sectional view and FIG. 114 is a plan view from the front surface. A spectacle lens 21 has a rotationally-symmetrical aspherical front surface 22 and a spherical back surface 23. A framing reference point 24 is defined to be coincident with a pupil of an eye 5 of a user when the spectacle lens 21 is installed on a frame. In the drawings, a z1-axis is defined to be coincident with a normal to the front surface 22 at the frame reference point 24, and x1- and y1-axes, which intersect at right angle, are defined in a plane that contacts with the front surface 22 and is perpendicular to the z1-axis. The y1-axis is direction from the base to the apex of the prism and the x1-axis is perpendicular to both of the y1- and z1-axes in a left-hand coordinate system.
The front surface 22 does not tilt with respect to the x1-y1 plane, while the back surface 23 tilts with respect to the x1-y1 plane. As a result, the spectacle lens 21 has a prismatic power whose base setting is the minus direction of the y1-axis.
However, since the above-described conventional spectacle lens is designed through the use of the front surface 22 and the back surface 23 that are originally designed for a lens having no prismatic power and it produces the prismatic power by tilting the front and back surfaces with respect to each other, although it can correct hereophoria, aberration caused by adding the prismatic power is not taken into consideration.
It is therefore an object of the present invention to provide an aspherical spectacle lens, which is capable of having a sufficient optical performance even if the lens has a prismatic power to correct hereophoria of an eye.
For the above object, the present invention is characterized in that aberration caused by adding a prismatic power is corrected by a rotationally-asymmetrical surface. The aberration caused by adding the prismatic power is rotationally-asymmetrical and therefore, it is difficult to correct the aberration with a rotationally-symmetrical surface. According to the present invention, one of front and back surfaces is formed as a rotationally-asymmetrical aspherical surface, which can correct the aberration caused by adding the prismatic power.
In the case when the back surface is rotationally-asymmetrical, it is preferable that curvature of an intersection line of a plane containing the normal to the rotationally-asymmetrical surface at a framing reference point and the rotationally-asymmetrical surface at the prism base side is larger than that at the apex side. The framing reference point is coincident with a pupil position of a user when the spectacle lens is installed on a frame, and is coincident with a prism reference point for a lens having a prismatic power. On the other hand, when the front surface is rotationally-asymmetrical, it is desirable that the curvature of the intersection line at the prism base side is smaller than that at the apex side.
In more detail, the following condition (1) is preferably satisfied within the ranges of 10xe2x89xa6hxe2x89xa620 and 30xe2x89xa6xcex8xe2x89xa6150;
C2xe2x88x921(h, xcex8+180)xe2x88x92C2xe2x88x921(h, xcex8) greater than 0xe2x80x83xe2x80x83(1) 
where
C2xe2x88x921(h, xcex8)=C2(h, xcex8)xe2x88x92C1(h, xcex8);
C1(h, xcex8) is curvature of an intersection line of a plane, which contains a z1-axis and forms angle xcex8 (degree) with respect to an x1-axis, and the front surface at a point whose distance from a z1-axis is h (mm);
C2(h, xcex8) is curvature of an intersection line of a plane, which contains a z2-axis and forms angle xcex8 (degree) with respect to an x2-axis, and the back surface at a point whose distance from a z2-axis is h (mm);
z1-axis is a normal to the front surface at the framing reference point;
y1-axis is direction from the base to the apex in a plane perpendicular to the z1-axis;
x1-axis is perpendicular to both of the y1- and z1-axes in a left-hand coordinate system;
z2-axis is a normal to the back surface at the framing reference point;
y2-axis is direction from the base to the apex in a plane perpendicular to the z2-axis; and
x2-axis is perpendicular to both of the y2- and z2-axes in a left-hand coordinate system.
Further, it is preferable that the condition (2) is satisfied when the back surface is rotationally-asymmetrical and that the condition (3) is satisfied when the front surface is rotationally-asymmetrical;
C2(h, xcex8+180)xe2x88x92C2(h, xcex8) greater than 0xe2x80x83xe2x80x83(2) 
C1(h, xcex8+180)xe2x88x92C1(h, xcex8) less than 0xe2x80x83xe2x80x83(3) 
Further, in order to respond to various combinations of spherical power, cylindrical power, cylindrical axis direction, prismatic power and base setting, it is desirable that semifinished lens blanks whose front surfaces are finished are stockpiled and a back surface of the selected semifinished lens blank is processed according to the customer""s specification in order to shorten delivery times.